Completion of boreholes by casing and cementing is well known. Following drilling of the borehole, a tubular casing, typically formed from steel tubes in an end to end string is placed in the borehole and cement is pumped through the casing and into the annulus formed between the casing and the borehole wall. Once set, the cemented casing provides physical support for the borehole and prevents fluid communication between the various formations of from the formations to the surface (zonal isolation). However, problems can occur if drilling mud remains in the borehole when the cement is placed, or microannuli form around the casing and/or borehole wall. The effect of these can be to provide fluid communication paths between the various formations or back to the surface and consequent loss of zonal isolation.
Sections of annulus can also be isolated by the use of packers. These are typically flexible bladders that can be inflated by pumping a fluid into them so that they expand and seal against the borehole wall. One common type of packer, the external casing packer (ECP) is inflated by pumping cement into the bladder where it is allowed to set and form the local seal.
Packers also suffer from problems. For example, they can fail to inflate or hold their inflation; they can be damaged during installation so that inflation is not possible; they are expensive and unreliable.
There are certain devices used in casing cementing operations that assist in trying to avoid the problems mentioned above. Centralizers for holding casing in place, are well known. A schematic view of a known centralizer is shown in FIG. 1 and comprises a pair of collar sections 2, 4 that are located around the casing (nor shown). The collars 2, 4 are connected by arms 6 that are bowed so as to extend away from the casing. In use, the bowed arms 6 bear on the borehole wall and hold the casing centrally in the borehole at this location. This helps ensure a regular annulus that can be filled with cement evenly to try to provide a good seal. Turbolizers are devices for attachment to casing that interact with fluid (cement) flowing in the annulus to provide uniform placement of cement while it is being placed. FIG. 2 shows a schematic view of a known tubolizer which, like the centralizer shown in FIG. 1 comprises a pair of collars 8, 10 connected by bowed arms 12. However, in this case, a number of shaped fins 14 are connected to the arms 12. In use, the turbolizer is placed on the casing and as cement is pumped through the annulus in a cementing operation, the fins modify the flow so as to assist in more uniform placement of the cement and avoid irregular flow.
The problems discussed above in relation to cementing for zonal isolation discussed above can occur even when using devices such as centralizers and turbolizers. It is therefore an object of the invention to provide methods an apparatus that can help avoid these problems.